Side band suppressed tuning for radio receiving systems



May 30, 1939. F. M. SCHMIDT 2,160,566

v SIDE BAND. SUPPRESSED' TUNING FOR RADIO RECEIVING SYSTEMS Filed May 23, 1932- 5 WW&,M

Patented May 30, 1939 UNITED STATES PATENT OFFICE SIDE BAND SUPPRESSED TUNING FOR RADIO RECEIVING SYSTEMS corporation of Ohio Application May 23, 1932, Serial No. 613,064

1 Claim.

This invention relates generally to radio receiving systems and particularly to improvements in the tuning arrangement thereof for controlling the output of the system.

One of its objects is to provide a receiving system of this character which has been designed to obtain radio reception only when the receiver is tuned in substantial resonance with the carrier signal being collected or so close to re the carrier that the quality of reproduction of music or other program in the loud speaker is indiscernably different from that which is reproduced when tuned exactly to the carrier.

Another object of the invention is the provii'5 sion of a positive, reliable and fool-proof receiving circuit wherein the set is maintained inoperative or silenced not only between station settings but also within the side bands of a given broadcast channel, and is rendered operative only when tuned to substantially the carrier frequency or to those portions of the side bands immediately adjacent to the carrier and where good reception would normally be reproduced, thereby effectually eliminating the usual disturbing noises which exist between stations on an automatic volume control receiver and avoiding the objectionable swish and bad quality obtained when tuning into and out of a station.

The invention further resides in the system and features hereinafter described and particularly pointed out in the appended claim.

In the accompanying drawing: Figure 1 is a diagrammatic view of a superheterodyne receiving circuit embodying my invention. Figure 2 is a resonance curve diagram of a broad type tuning characteristic. Figure 3 is a similar view of the frequency characteristic of sharply tuned circuits.

Similar characters of reference indicate corresponding parts throughout the several views.

By way of example, I have shown my invention in connection with a straight circuit of an ordinary superheterodyne receiver with an automatic volume control, the whole being of any suitable and well known construction as used in the art.

Referring now to Figure 1, signal energy received by the customary absorption or antenna structure is impressed upon the input or preselector circuit Act a radio frequency amplifier tube B and preferably again amplified through intermediate frequency circuits including one or more similar tubes B1, B2, B3, etc., associated with six tuned frequency circuits. The letters C and C1 indicate detector tubes.

Immediately preceding the second detector tube C1 is a tuned circuit D preferably loosely coupled with a tuned circuit D1 to give a sharp resonance curve. Circuit D1 is also coupled to a coil D2 and the coupling so arranged that, in combination with the preceding circuits identified as B4 and B5, the desired resonance curve is obtained for the production of good fidelity. Included in the tuned circuit D is a tube E which is biased beyond cut off and upon the impression of a signal allows current to flow in its plate circuit. This tube is of the type having the usual vacuum tube voltmeter action allowing'w current to flow through the resistance E1 in its plate circuit and in turn causing the grid bias of tube F to be driven so far negative that it cuts off the tube F and allows no plate current to flow in the plate circuit of the latter. The 20 plate circuit of said tube F is fed through the, grid coupling resistance R of the first audio tube G, and when the tube F is drawing no plate current, the first audio tube has normal bias voltage and is in normal operatingcDr-Ldi- 25 tion and the set adjusted for reception. When no signal is being received on the antenna, the constants and voltages of the circuits associated with the'tubes F and G cause the tube G to be biased beyond cut off and therefore renders tube G inoperative. Obviously, the condition required to put the audio tube G in an operative state is to have a radio frequency voltage applied to the circuit D. Owing to the fact that this circuit is preferably loosely coupled to the circuit D1 of the last tuned frequency circuit, the selectivity is made sharper than the overall selectivity of the set, including the coil D2 feeding the second detector tube C1, and only releases the first audio tube G for condition of am- 40 plification when tuned almost dead on the carrier.

The six tuned intermediate frequency circuits including the tubes B1, B2, etc., preceding the second detector C1, and the pre-selector tuned circuits A are so arranged that the overall resonance curve is sufliciently broad at the peak to insure good fidelity and is as indicated in Figure 2. The coupling of the circuit D with the circuit D1 causes the tuning characteristic of the voltage applied to the grid of tube E to be sharp as seen in Figure 3. Owing to the shape of the tuning curve in Figure 2, the tone quality remains uniform over the range of selectivity asup sociated with the tuned circuit D and impressed upon the control grid of the tube E, thus allowing the receiving set to become operative only on that portion of the broadcast channel where good reception is normally obtained.

As this is a superheterodyne receiver, the method .of receiving a signal would be to adjust the pre selector circuits A and B and the oscillator circuit C, so that the heterodyne between the desired signal, being resonated by circuits A and B, is being converted to the intermediate frequency to which circuits B4, B5 and circuit 131 are tuned. Owing to the additional selectivity caused by the type of couplingbetween circuit D and circuit D1 sufficient voltage is impressed upon the control grid of tube E only when the pre-selector circuits are adjusted to the carrier or to the side bands immediately adjacent to the carrier. The voltage impressed upon the tube E allows plate current to flow in its plate circuit through resistance E1. The current flowing through E1 biases tube F negatively and beyond cutoff, which shuts 'oif the plate current fiowiug in the plate circuit of tube F. As this current was flowing through resistance R, the grid coupling resistance of tub e G, by virtue of the constants'of the circuit, had caused tube G to be biased beyond cut off. Elimination of this current allows the bias of tube G to assume a normal bias and also allows tube G to amplify. While the conditions above obtain in these circuits, voltage is likewise impressed on C1 by virtue of the coupling from circuit D1 to circuit D2, and further, by virtue of that same coupling, the broadness of the resonance curve allows substantially uniform voltage to be impressed upon 01, which inturn results in only perfect reception being heard 'from the receiver.

'By this system of controlling the output of the receiving set, the combination of a broad top tuning characteristic is obtained in the main frequency tuned circuits with additional selectivity which becomes very narrow or pointed at thtop, this sharp or pointed selectivity being the determining factor as to when the first audio tube is released so that it can amplify, whereas the wider top selectivity characteristic determines the quality'and fidelity of the receiver as a whole. Aside from effectively eliminating static and interference noises so prevalent between stations in receiving sets employing automatic volume control, as well as dispensing with the bad quality of reception on the side bands of the carrier, this invention does not require the use of any indicator meter to inform the user or operator when the best quality of reception is obtained. With'this system, when the tuning knob of the set is adjusted for a given station setting, the set is maintained absolutely silent unless tuned to a carrier frequency or to its immediately adjacent side bands, and it is only when the-receiver is tuned substantially dead on the carrier that reception can be had. By virtue of eliminating all objectionable noise and interference and maintaining the receiver silent or inoperative between stations as well as within the more remote side bands, and permitting reception only when tuned to the carrier frequency, or adjacent its side bands, this circuit arrangement enables receivers of extreme sensitivity to be made without the usual tuning objections.

1 claim as my invention:

In a radio receiving circuit, a signal collecting device, a signal selection circuit, and means associated with said signal selection circuit for rendering the output system of the receiver inoperative when' the selection circuit is resonant with a frequency at which no signal is impressed onsaid collecting device and for maintaining the output system inoperative when the signal selection circuit is adjusted to resonance with the more remote side bands of any signal impressed upon said collecting device, said means,

comprising loosely coupled tuning circuits including audions, one of said audions having its control grid connected to one of the tuning circuits and its plate connected to the control grid of the other audion, and an interposed resistance connected to bias one of the audions negatively beyond cut off to shut off the plate current in the other audion, and an audio tube having its control grid connected to the plate of said secondnamed audion and including a grid coupling resistance for energizing the plate current of said last-named audion.

FRANCIS M. SCHMIDT. 

